VENTRAL STRUCTURE OF TRIARTHRUS 205 



few cases the evident sliding or displacement of the dorsal 

 and ventral surfaces further complicates the attempt to refer 

 the appendages to definite divisions of the animal. 



Paired Uniramose Appendages. 



Anterior Antennae, or Antennules. — These have been de- 

 scribed by ilatthew, Walcott, and the writer (I. c). Wal- 

 cott showed their proximal extremities and their mode of 

 attachment at the side of the hypostoma. Little more can 

 now be added except that they are evidently the first pair 

 of antennal organs, and correspond to the antennules of other 

 Crustacea. The strong basal joint or shaft is shown in Plate 

 VIII, figures 9, 10, 11, attached to the ventral side of the 

 head at each side of the hj^DOstoma, near the middle of its 

 length. The shaft carries a single flagellum, and thus agrees 

 with the typical uniramose antennule of the nauplius of Crus- 

 tacea. This simple antennule is still present in the Isopoda, 

 as in Mannuopsis typica. The flagella curve forward and ap- 

 proach, nearly touching as they cross the doublure. Beyond 

 the limits of the head they are variously disposed, though 

 usually extending forward, at first diverging for half their 

 length and then slightly converging (Plate VIII, figures 

 5, 6, 7). 



Paired Biramous Appendages. 



The remaining paired appendages of the trilobite all seem 

 to be biramous, and agree closely in their general features. 

 Adjacent members of the series present very slight differ- 

 ences. It is only when the primitive and simple phyllopodous 

 legs of the pygidium are compared with the anterior thoracic 

 or cephalic appendages that variations of note can be ob- 

 served, although these are of form and not of structure. On 

 this account there is no well-defined separation into pos- 

 terior antennae, mandibles, maxillae, maxillipeds, thoracic, 

 and pleopodal appendages. It is most convenient, therefore, 

 to number them from before backward, and to indicate 



